Make sure it has a double truss rod by unscrewing the truss rod cover on the headstock and looking to see if it has 2 truss rods.
Make sure it has both input jacks, one saying standard the other being stereo.
I would look up and down the neck for cracks, if I where you also I would be researching whatever source you can via the internet or music stores that have vintage Rickenbackers, also if you can take a bunch of pictures before you buy and post them here on TalkBass for other Rick experts to look at and give you advise if that would be possible, that would be truly the way to go if I was you.
Hope this helps.

I'd ask to remove the truss rod cover and check the nuts - there should be no wear. In the old days, lots of techs didn't know how to adjust them. If the strings are round wounds, check for fret wear. I had a set on mine, and within a month, I saw wear. Some 4001's didn't like the round wounds at all. Mine was one of them.

Also check for tail lift - the bridge should be flat against the body. If it's slightly lifted at the rear, it's probably ok, if it's significant, the tail piece will most likely need to be replaced.

Since there are known non-original parts, I'd also ask to remove the pick guard. Check for poor solder joints and obvious replacement pots, caps, and wiring. The route from the neck pickup to the control cavity should look pretty rough - in those days they didn't route it, but instead, used a drill bit and made multiple connecting holes.

Check that the serial number is on the jack plate, and check for both a mono jack and stereo (rick-o-sound) jack.

If you have a smart phone, you can check the serial number right on the Ric website.

1973 was a transitional year, so there may be a checkered binding or not.

That is no '73 4001, more like '75 or later. 1" neck pickup location and the controls are in the late '70s to early '80s configuration. Some of the above comments are off base, too. Why are replaced parts a RED FLAG, for one thing? This is not uncommon with lots of old instruments, sometimes just for the fun of modding an instruments, sometimes because something got damaged and needed replacing, just for a couple of reasons. It looks like this pickguard was replaced for the cosmetics of this one being a mirrored one.

Truss rod nuts that have some wear on them is not a big deal, either, they sometimes need to be adjusted and can get a bit worn looking over time if done often enough. Even if there is some fingerboard separation that is not as big a deal as a lot of guys here (probably armchair experts) seem to think. There are lots of examples out there of this and even some serious cracks running down the necks that have been repaired and are as strong as new, really. Do learn how to adjust these rods correctly and most likely, once adjusted for the strings of your choice, will never have to be adjusted again.

I do take pause with the seller's comment:

Some "belt-buckle rash" on the back.

Click to expand...

Some?!?!?!?!?!?!?

That gives me some pause if only because he has just slightly under exaggerated his description of the condition of the back of the bass. He also failed to mention the Fender style thumb rest that has been added to the bass.

If you remove the TRC to check the truss rod nuts be careful when screwing it back down:

DO NOT OVER TIGHTEN THE SCREWS! You will crack the plexi. If that does happen you will owe the seller quite a chunk of change.

These '70s 4001s are pretty robust instruments unless messed with by an ignorant owner or "tech".

The seller shouldn't be unwilling to do this, it's part of any standard authenticity verification of a Rick bass.
Looks authentic, but it is likely later production than the seller claims.
Use this to your advantage if it is the case.

Here's a helpful site for info on how to spot a fake, and when you do buy it, how to set up and maintain it.http://joeysbassnotes.com/

And yeah, if it's in good working order, that's a pretty decent price.

October '73 would have an "MJ" serial number. Of course, jack plates are interchangeable. The 1" neck pickup spacing and the vol/tone knob configuration might just be an artifact of the replaced pickguard. Guards with the 1/2" spacing are less common, so if the old guard were cracked, a 1" spacing guard might have been all the owner could find. It's possible some routing may have had to have been done for the 1" placement, so I'd look under the guard as well.

Still, $900 is a pretty good deal if the neck and truss rods are in good shape.

Again, thanks all. I feel pretty good about this whole thing, so long as every thing looks good "under the hood." I do have a smart phone, so I'll run the serial number on site. I'll also have some screwdrivers in my car in case the seller can't find any. FWIW, he seemed like a very nice man over the phone.